In order to reduce the emission of carbon dioxide, an increasing number of today's vehicles have an idle-stopping function, i.e., the function of stopping the engine when the vehicle stops, and restarting the engine to move the vehicle again. Among such vehicles, vehicles equipped with an integrated starter generator (ISG) are especially gathering attention, because the ISG is capable of quietly restarting the engine, and also capable of regenerative braking.
An ISG is one of the automobile engine accessories, and functions as a generator to generate electricity from the driving force of a crankshaft while the engine is running in a steady operation, and as an electric motor (starter) to forcibly rotate the crankshaft, kept at a standstill, to start the engine.
FIGS. 5 and 6 exemplify a belt transmission device for a vehicle equipped with an ISG. This belt transmission device includes a crank pulley 51 mounted to a crankshaft 50; an ISG pulley 54 mounted to the rotary shaft 53 of the ISG 52; an engine accessory pulley 57 mounted to the rotary shaft 56 of an engine accessory 55 such as a water pump; a belt 58 trained around these pulleys 51, 54 and 57; and a tension adjusting device 60 for keeping the tension of the belt 58 in an appropriate range.
The tension adjusting device 60 includes a tension pulley 61 kept in contact with the belt 58 on the downstream side of the crank pulley 51 and on the upstream side of the ISG pulley 54; a pulley arm 62 supporting the tension pulley 61; and a hydraulic auto-tensioner A configured to bias the pulley arm 62 in the direction in which the tension pulley 61 is pressed against the belt 58. One end of the hydraulic auto-tensioner A is coupled to the pulley arm 62, and the other end of the hydraulic auto-tensioner A is coupled to an engine block 59. The pulley arm 62 is supported to be pivotable about a fulcrum shaft 63 fixed to the engine block 59.
As illustrated in FIG. 5, while the engine is running in a steady operation, the crankshaft 50 rotates as a driving shaft such that the rotation of the crankshaft 50 is transmitted through the belt 58 to the pulleys 54 and 57, thereby rotating the rotary shaft 53 of the ISG 52, and the rotary shaft 56 of the engine accessory 55. At this time, the portion of the belt 58 where the tension adjusting device 60 is located is being pushed out from the driving pulley (crank pulley 51), and thus becomes the loose side of the belt 58.
On the other hand, as illustrated in FIG. 6, when restarting the engine after the idle stop, the rotary shaft 53 of the ISG 52 rotates as a driving shaft such that the rotation of the rotary shaft 53 is transmitted through the belt 58 to the pulley 51, thereby rotating the crankshaft 50. At this time, the portion of the belt 58 where the tension adjusting device 60 is located is being pulled by the driving pulley (ISG pulley 54), and thus becomes the tension side of the belt 58.
Since, as described above, a belt transmission device equipped with an ISG is configured such that the crank pulley 51 functions as a driving pulley while the engine is running in a steady operation, and the ISG pulley 54 functions as a driving pulley when the engine is started by driving the ISG, the portion of the belt 58 with which the tension pulley 61 of the tension adjusting device 60 is in contact becomes the loose side of the belt 58 while the engine is running in a steady operation, and becomes the tension side of the belt 58 when the engine is started by driving the ISG.
Japanese Unexamined Patent Application Publication No. 2009-275757 (JP '757) discloses a hydraulic auto-tensioner used to maintain the tension of a belt for driving engine accessories, and including a vertically extending sleeve; a rod vertically movably inserted in the sleeve; a spring seat fixed to the top end of the rod; and a return spring configured to bias the spring seat upwardly. The return spring moves the rod, relative to the sleeve, to the position at which the tension of the belt are equal in strength to the biasing force of the return spring, thereby applying initial tension to the belt.
The hydraulic auto-tensioner of JP '757 further includes, as a mechanism for generating a damping load, a pressure chamber defined inside of the sleeve such that the volume of the pressure chamber increases as the rod moves upwardly relative to the sleeve, and such that the volume of the pressure chamber decreases as the rod moves downwardly relative to the sleeve; a reservoir chamber defined outside of the sleeve; oil stored in the pressure chamber and the reservoir chamber; oil passages through which the lower portion of the pressure chamber communicates with the lower portion of the reservoir chamber; a check valve allowing oil to flow, through the oil passages, only from the reservoir chamber to the pressure chamber; and a leakage gap defined between the inner periphery of the sleeve and the outer periphery of the rod.
When the tension of the belt decreases, the rod moves upwardly relative to the sleeve, thereby absorbing the looseness of the belt. At this time, since the pressure in the pressure chamber falls below the pressure in the reservoir chamber, and thus the check valve opens, oil flows from the reservoir chamber to the pressure chamber through the oil passages, thereby enabling the rod to move in a smooth manner.
On the other hand, when the tension of the belt increases, the rod moves downwardly relative to the sleeve, thereby absorbing the tension of the belt. At this time, since the pressure in the pressure chamber increases, and thus the check valve closes, no oil flows in the oil passages, and oil in the pressure chamber flows out through the leakage gap, which is defined between the inner periphery of the sleeve and the outer periphery of the rod, so that a damping load is generated due to the viscous resistance of the oil flowing in the leakage gap.
If the above-described conventional hydraulic auto-tensioner of JP '757 is used in the belt transmission device in FIGS. 5 and 6, which is a device for a vehicle equipped with an ISG, it is difficult to obtain an optimum damping load both while the engine is running in a steady operation, and when the engine is started by driving the ISG.
Namely, in a belt transmission device for a vehicle equipped with an ISG, as described above, the portion of the belt which becomes the loose side of the belt while the engine is running in a steady operation becomes the tension side of the belt when the engine is started by driving the ISG.
Therefore, if the magnitude of the damping load generated in the hydraulic auto-tensioner is set to be suitable for the steady operation of the engine, when the engine is started by driving the ISG, the hydraulic auto-tensioner will be excessively pushed and contracted, thereby generating slippage between the pulleys and the belt. This might cause the belt to squeal and/or shorten its service life, and also might make it impossible to start the engine in the worst case.
On the other hand, if the magnitude of the damping load generated in the hydraulic auto-tensioner is set to be suitable for when the engine is started by driving the ISG, the tension of the belt will be unnecessarily large while the engine is running in a steady operation. This will cause large energy loss due to the friction between the belt and the pulleys, thus increasing the fuel consumption of the engine.
It is an object of the present invention to provide a hydraulic auto-tensioner used in a belt transmission device for a vehicle equipped with an ISG that is capable of keeping the tension of the belt low while the engine is running in a steady operation, and effectively preventing the slippage of the belt when the engine is started by driving the ISG.